Electrostatic spray coating system



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BELL EDGE KV Aug. 7, 1962 J. W. JUVINALL ETAL ELECTROSTATIC SPRAYCOATING SYSTEM Filed Jan. 8, 1959 4 Sheets-Sheet 1 JAMES W. JUVINALLJAMES C. MARSH INVENTORS Aug- 7, 1962 J. w. JUVINALL ETAL 3,048,498

ELEcTRosTATIc SPRAY comme SYSTEM 4 Sheets-Sheet 2 Filed Jan. 8, 1959INVENTORS JUVINALL a2ac-KW A fameys JAMES W. BY JAMES C Aug. 7, 1962 J.W. JUVINALL ETAL ELECTROSTATIC SPRAY COATING SYSTEM Filed Jan. 8, 1959 4Sheets-Sheet 3 A H'ar y Aug. 7, 1962 Filed Jan. 8, 1959 J. W. JUVINALLEVAL ELECTROSTATIC SPRAY COATING vSYSTEM /lOa/ 4 Sheets-Sheet 4 Dc 00m/rINVENT OR.

JAMES W JUV/NALI. JAMES G MARSH Arfom y United States Patent 3,948,493ELECTRGSTATIC SERAY CQATING SYSTEM James W. Juvinall and .lames C.Marsh, Indianapolis,

Ind., assignors to Ransburg Electro-Coating Corp., Indianapolis, Ind., acorporation of Indiana Filed lian. 8, 1959, Ser. No. 785,754

27 Claims. (Cl. 117--93) This invention relates to spray coating with4 aliquid coating such as paint, and more particularly to spray coatingsystems wherein electrostatic forces are utilized to aid in thedeposition of spray particles on the article to be coated.

This application is a continuation-in-part of our application Serial No.572,752 filed March 20, 1956 and now abandoned.

It is the general object of the present invention to produce new andimproved electrostatic spray coating apparatus and methods.

Still another object of the invention is to produce an electrostaticspray painting system in which the average potential gradient of theelectrostatic depositing eld may be maintained at predetermined desiredvalues in spite of varying distances between the article being coatedand the charging electrode, between which article and electrode thedepositing field extends.

Yet another object is `to maintain the average potential gradient of anelectrostatic atomizing field at or about the optimum atomizinggra-dient throughout substantial variations in the spacing between theatomizing zone and the other field electrode.

A further object of the present invention yis to provide anelectrostatic spray painting system which includes a charging electrodefor creating an electrostatic depositing field extending from theelectrode to the article being coated and which -is incapable ofproducing dangerous disruptive discharges, even when the chargingelectrode is closely approached by the article being coated or bypersonnel.

Electrostatic spray coating systems have been successfully used incommerce in several `different forms, for example, systems have beensuccessfully employed which utilize spraying or atomizing devicesemploying air as the atomizing medium. Such systems normally include anindependent electrode in the form of a .grid of wires which ismaintained at high potential, thereby establishing an electrostaticdepositing field extending between such electrode and atleast one of thearticles being coated as the other electrode. In such systems, theconventional air spray gun projects the atomized particles into theelectrostatic field, with the result that the atomized particles becomeelectrostatically charged with respect to the article and thereby arecaused to be deposited thereon. As an example of this type ofelectrostatic spray coating system, reference may be had 4to UnitedStates Letters Patent No. 2,3 34,648.

Another system of electrostatic spray .painting widely used commerciallyobviates the necessity of air as an atomizing agent, one form of thissystem effecting atomization by electrostatic forces. In this form, theatomizing device is maintained at high electrical potential, and thusserves as the charging electrode for electro- 3,048,498 Patented Aug. 7,1962 ICC , application of Edwin M. Ransburg, Serial No. 771,505,

statically charging the spray particles. =In systems where filedNovember 3, 1958 and now Patent No. 2,893,894, a continuation ofapplication Serial No. 143,994, filed February 13, 1950V and nowabandoned, or to James W. Iuvinall Patents No. 2,764,125 and No.2,764,712, both issued September 25, 1956. Other forms of this systemmay also be used, such as, for instance, one where the coating materialis atomized centrifugally or by the application of pressure to theliquid. In this type of system, the atomizer itself may constitute acharging electrode, or the particles so atomized may be projected intoan electrostatic depositing field maintained between a separate chargingelectrode and the article.

It will therefore be seen lthat various -means for effecting atomizationmay be utilized in electrostatic spray coating systems, and that in suchvarious systems, the charging electrode may be either the atomizeritself or a separate charging electrode.

Whereas devices of the type referred to readily lend themselves tocommercial usage and are capable of greatly reducing the cost of spraypainting through the avoidance of coating material losse-s and thereduction in other costs, including labor of coating, electrostaticspray coating systems have suffered certain 'limitations in usage whichhave prevented wider adoption and usage thereof. One of the limitationsreferred to is the fact that proper electrostatic atomization andefficient deposition of spray coatings have required the employment ofan electrostatic field established by relatively high effectivevoltages, as of the order of one hundred thousand volts, which have manycritical and unexpected characteristics, and resent diiiicultiesrelative to safety both as to fire and as to shock hazard to personnel.

In the type of system employing electrostatic atomization, it has beenfound that there is an upper as well as a lower limit on the voltagesthat should be employed, an unexpected characteristic of high voltagefields and electrostatic atomization thereby. Where electrostatic forcesare employed to effect the atomization of the coating, it is importantto maintain the potential gradient of the electrostatic field extendingfrom the atomizing device to the article within predetermined limits,which limits define the range of the optimum atomization of theatomizing device. Thus, while it had been thought that fineness ofatomization and the quality of the 'paint finish produced would beimproved by increasing the average potential gradient of the field tothe maximum consistent with safety, it has now been discovered that thequality of atomization does not continue indefinitely to improve withincreases in the average potential gradient, but rather that there is,for any given set of other conditions, a rather definite potentialgradient at which the quality of atomizat-ion is optimum, and that thepotential gradient resulting in atomization of optimum quality is eitherincreased or decreased appreciably, the .atomization Will suffer. It hasbeen found that if, for any given set of conditions, the averagepotential gradient is progressivelyincreased from a low value, the cuspsinto which the field deforms the edge of the film of coating material onthe atomizer, become more closely spaced and atomization becomes finer,i.e., improves in quality. During this initial increase in potentialgradient, the cusps continue to be equally spaced and stable, buteventually a condition is reached wherein further increases in fieldgradient cause the cusps to become irregular and unstable. Specifically,the cusps seem to join together in some cases and to flail about 4inothers, perhaps under the influence of increasing mutual electrostaticrepulsion; an-d instead of the cusps breaking up at their tips uniformlyand stably into spray particles of appropriate size, many objectionablelarge .particles are formed.

Because of the foregoing, it has been necessary to andasse maintainrelatively definite and tiXed spatial relations between theelectrostatic atomizing device and the article being coated, in orderthat the potential gradient may be kept Within desired limits. Anycondition which resulted in changes of the atomiZer-article spacingcaused corresponding changes in the potential gradient, and resulted indeterioration of atomization if the changes in potential gradientexceeded the limits described.

In the type of system which does not employ an electrostatic eld foreffecting the atomization of the coating, but rather relies uponmechanical means for atomization, such as compressed air or liquidpressure, it is found that the higher the negative voltage on thecharging electrode, the higher the coating efficiency. Thus, it isadvisable to employ voltages on the charging electrode as high as 150kv., or even more. As in the case of the electrostatic atomizer,however, the voltages actually employed are dictated at least in part bythe ability to avoid situations where the electrode and the articlespacing cannot be maintained within quite definite predetermined limits.y

As many of the spray coating materials employed are inilammable, and insome cases highly so, it has at all times been necessary to provide aminimum spacing between the electrodes in order to avoid the likelihoodof sparks or arcs of suliicient intensity to initiate a fire orexplosion. These considerations have dictated the necessity of confiningthe use of the various systems now employed in commercial practice toinstallations wherein the charging electrode is maintained in a fixedposition with relation to the articles to be coated. As it is customaryin commercial installations to have the articles suitably mounted on atraveling conveyor so that they are successively moved to pass thelocation where one or more atomizers are provided, it has been thepractice to station the charging electrode in fixed relation to the pathof movement of the articles. In this way the necessary distance may bemaintained with sutiicient accuracy to assure the avoidance ofdisruptive discharges, such as might cause a lire or explosion, providedthe electrode is spaced far enough from said Ipath to prevent diiculty.Suiciently more than the minimum sparking distance must be allowed toprovide for anticipated variations in the spacing, as might result froma bent hanger, or swaying of the articles. In those instances wheremechanical atomization is employed and a separate charging electrode isprovided, it has been the practice to space the charging electrode fromthe article a distance which is twice the sparking distance. Maintainingthis distance results, of course, in a decrease in efficiency of theoperation, as the average potential gradient is necessarily reduced.This allowance of additional spacing for safety purposes, however, hasbeen dictated by the practical necessities which result in variations inelectrode-article spacing during normal operation of a system of thisnature. For instance, many objects carried on conveyors of the typereferred to are subject to some movement due to swaying, or may lie in adifferent plane due to a bent hanger, as mentioned above, which causesthe articles to. move first closer and then farther away from thecharging electrodes. In other instances, articles of irregular shape maybe rotated while being sprayed, and due to their irregular shapes andthe rotation, certain portions of their surfaces are disposedappreciably nearer to the charging electrode than other portions of thesurfaces at other times during the spray coating.

Even in cases where the coating material employed is such as not tocreate a danger of fire or explosion, the necessity of avoiding'disruptive discharges from the charging electrodes is still present.Close approach of personnel to the charging electrode will result in asevere and unpleasant shock, even though the current dow is maintainedbelow lethal levels. Thus, it is also important to avoid disruptivedischarges or this type, of such i intensity as to be severely ordistressingly shocking, for the protection and comfort of personnelworking in the area.

From the foregoing, it will be realized that the systems described haverequired the utilization of means for maintaining within predeterminedlimits the spacing between the electrodes creating the electrostaticteld, i.e., the spacing between the charging electrode and the article.This requirement has mitigated against the provision of an unrestrainedhand-held spray device in an electrostatic coating system. This isapparent because an operator using-such a device would undoubtedlychange the spacing between the electrodes, and thus change the potentialgradient of the electrostatic lield.

lt has now been discovered that systems of the type referred to can beprovided in which the variations in spacing between the electrodes whichmight occur in use, including the variation in spacing that would beeffected through the use of a hand-held atomizing device, can beautomatically compensated for to an extent which makes it possible tomaintain the potential gradient of the held at values consistent withhigh depositing ethciencies (and with high atomizing eiciencies, insystems where electrostatic atomization is employed) throughout y arange of spacings extending from the greatest down to the least employedin spraying and which also makes it possible to insure that anygap-bridging discharge occurring at or below such least distance will beof unobjectionable intensity.

Two characteristics may be built into the system and may be employedconjointly to bring about the results referred to immediately above. Thetirst characteristic is the use of an impedance oi' predeterminedmagnitude in the electrical system between the source of high voltageand the air gap between the charging electrode and the article. lf theimpedance employed is sufficiently high with relation to the potentialemployed and the air gap commonly present, variations in the air gapwhich would normally have a direct and major iniluence on the potentialgradient will result in voltage variations such as will satisfactorilycompensate for such changes in spacing. Thus changes in potentialgradient resulting from changes in spacing are greatly minimized and apredetermined desired held gradient can be maintained in spite of widechanges in the distance from the article to the charging electrode."Furthermore, this compensating action can be established to such adegree to assure that at all times variations in the air gap between theelectrodes will not result in undesirably low lield gradients.

The second characteristic which may be employed is the provision at thelocation of the air gap of a charging electrode or electrode system oflow effective capacity. When we state herein that an electrode systemhas a low effective capacity we mean that the electrode system has suchlow true capacitance, such poor conductivity of the material of which itis formed, or such shape (particularly as to sharpness or bluntness ofits configuration) that, in an electrostatic coating system of thecharacter with which we are here concerned, the energy contributed to adisruptive discharge between such electrode system and an opposed bluntelectrode by the electricity stored in the electrode system isinsuticient to render such discharge objectionable. We have found thatif some portion of the charging electrode or of the system between thecharging electrode and the resistance previously referred to is ofhighly conductive material, such as metallic wire or the like, such canbe tolerated, provided that it is of such limited area that it has nosubstantial capacitance. In other words, if Ahighly conductive materialsare to be used adjacent the air gap, their actual capacitance must be solow as to be within tolerable limits. if, on the other hand, theircapacitance is greater than the tolerable amount, they must be of suchlow conductivity that the energy thereof cannot be discharged in such amanner as to produce anl objectionable disruptive discharge.

In earlier commercial systems of the types previously mentioned, it hasbeen the practice to equip the high voltage source with an effectiveimpedance of about megohms in the power pack itself and in series withthe hot terminal. This impedance was quite ineffective to prevent acurrent flow, in the event an article or other object too closelyapproached the electrode, which would give an objectionable disruptivedischarge. In addition, the capacitances and conductances of theelectrodes used were such as to permit an objectionable disruptivedischarge between the electrode and the article upon a close approach ofthe article thereto.

While there are patents in various iields where high voltage is usedwhich, like the impedances embodied in the high voltage sourcesdescribed above, incorporate resistances to prevent lethally heavyshocks or severe arcing over upon the close approach of a person orobject to the discharge electrode, none of such patents is in any wayconcerned with a system for automatically maintaining withinpredetermined limits the average potential gradient between thedischarge electrode and the article in spite of wide Variations in thedistance or air gap existing between the article and the electrode andin the presence of an essentially constant high voltage applied by thehigh voltage source, and which system, when cou-pled with an electrodehaving a capacity or conductivity insuiiicient to release a heavy charge(i.e., a suitable effective capacity), provides an electrostatic spraycoating system incapable of producing objectionable disruptivedischarges even when closely approached by the article being coated, orby an operator. For examples of the patents referred to above, referencemay be had to the following United States Letters Patent: Chapman878,272; Wintermute 1,913,784 and 1,968,330; Bennett 2,295,152; andMelton 2,187,624. Moreover, efforts have heretofore been made to solvesuch problems in the electrostatic spray-coating iield, reference beingmade to U.S. Patents 2,509,277 and 2,526,763, British Patent 688,788,and German application K 3942; but none of these did succeed in solvingthe above-mentioned problems.

As previously mentioned, atomizing devices embodying features of thepresent invention may be constructed in a manner which will maintain apredetermined range of potential gradient between an electrostaticatomizing head and an article, regardless of changes in the spacetherebetween, and wherein dangerous and objectionable discharges may beprevented, even -where the high potential eiectrode is accidentallybrought into contact with the operator or with the article being coated.By reason of the foregoing, it is possible to produce a hand-held ormanually manipulatable electrostatic atomizing device which is capableof the high eiciency performance of the electrostatic atomizing devicespreviously mentioned, yet may be used with perfect safety as a hand-heldatomizer, and to materially improve systems utilizing a ixedly mountedatomizer.

Other features and advantages of this invention will be apparent fromthe following description and the drawings, in which:

FIGURE 1 is a perspective view of a system for electrostaticallyatomizing and depositing paint on the articles to be coated;

FlGURE 2 is a longitudinal sectional view of the atomizing device shownin FIGURE 1;

FIGURE 3 is a fragmentary View, partly in section, showing in moredetail the means for rotating the head of the atomizing device and forsupplying liquid `coating material thereto for atomization;

FIGURE 4 is a chart graphically illustrating, by appropriate curves,certain voltage and distance relationships;

FIGURE 5 is a longitudinal sectional view of another form of atomizingdevice adapted to be held in the hand and used in a system of thegeneral character shown in FIGURE 1, and also having a rotatingatomizing member;

FIGURE 6 is a longitudinal sectional view of still another form ofatomizing device particularly adapted to be held in the hand,atomization in this case being elected by mechanical forces, highhydrostatic pressure of the paint supply in the form shown;

FGURE 7 is an elevational view of another system embodying ourinvention, atomization in this case being by a conventional air gun;

FIGURE 8 is a partial elevational View at rightanglesto that of FIGURE 7and along the line 8-8 thereof; `FIGURE 9 is a schematic view of yetanother system embodying our invention; and

FIGURE 10 is a circuit diagram of the power supply illustratedschematically in FIGURE 9.

Referring now to the particular embodiment of the invention illustratedin FIGURES l to 3, it will be seen that a succession of articles 10 aremoved through a coating zone, the articles being here illustrated asgarnish moldings for the window openings of automobiles, an articleditlicult to paint eiiiciently with conventional systernutiliZinghand-held air spray guns. The articles are here shown as beingmoved along to the right of the drawing as illustrated, by hangers 11 inturn connected to a chain (not shown) on a conveyor track 12 ofconventional overhead conveyor means, as illustrated. This movement iswithout rotation and the articles are shown as being painted on one sideonly (except for such Wraparound as may occur' due to the electrostaticaction), and it will be understood that in practice another man wouldnormaily be stationed on the other side of the line at a second coatingstation to provide a coating of the desired lm thickness on the otherside of the articles.

The atomizing device identified in general as 14 and to be more fullydescribed in detail hereafter, includes a lonlg handle portion 15 whichits outer housing of polvethylene or other non-conducting material withgood high voltage insulating characteristics, and a rotating bell-likehead 16 from the edge of which the liquid coating material iselectrostatically atomized. The charged spray particles of paint or thelike are then attracted to and deposited upon the articles under theinfluence of electrostatic forces, the articles normally being groundedand the hot terminal of the power pack 17 being connected to theatomizing edge in a manner hereinafter more fully described, andincluding a high resistance in the connection and close to the atomizinghead 16.

In the system illustrated (and as may be best seen by also referring toFIGURE 3) a supply tank 18 supplies paint through a tube 19 to a pump2t), preferably of a positive displacement Variable speed type driven byits own motor. The output of the pump is delivered through a paintsupply conduit 20a tc the opening in a hollow shaft of an electric motor22, this paint supply conduit continuing on up through a flexibleconduit arrangement indicated in general as k23 leadnig to the hand-heldatomizing device, the paint eventually issuing through a central openingin the bell-like rotating atomizer member 16 to fbe formed in a thinfilm on the inner surface of this member and to have the spray particlesatomized from such thin film at the edge of the rotating member, asmorel fully described and claimed in the E. M. Ransburg Patent No.2,893,894. As is more fully described in such patent, the high potentialelectrostatic field existing between the atomizing edge and the articlesdraws the iilm edge into closely adjacent cusps with the spray particlesor droplets issuing from the ends of these cusps during atomization.Except for the hand-held atomizirrg device and the high resistanceconnection, the system so far described is of a standard spaanse drive ahollow exible metallic shaft 2li surrounding a paint tube 21, the outersheath of the conduit assembly being a conduit 25 of polyethylene orother material having good high voltage insulating properties. Highvoltage, of at least 40,000 volts and preferably 70,000 volts or more,is supplied from the high voltage pack 17 (generally from the negativeterminal thereof) through the lead 26 to the motor ZZ, the liexiblemetallic drive tube 24 conducting the high voltage to the atorniZingdevice. The paint supply tank 18, the pump unit 2.0 comprising the pumpand its motor, and the rotator motor 22, are all mounted on a supportmember 27 car-fied on legs 28 of ceramic or other rigid material of goodhigh voltage insulating characteristics; and the pump motor and rotatormotor are supplied with drive current (as conventional 60 cycle ll()volt A.C.) through isolating transformers `with proper high Voltageinsulation so that the whole arrangement is maintained at high voltagewith respect to ground. While not illustrated, these parts wouldnormally all be enclosed in an insulating housing to protect againstaccidental contact of a person with any part at high voltage; and notonly the input of the high voltage supply tuiit but also the isolatingtransformer inputs would have conventional on-off switches therein, sothat the operator may terminate coating operation at any time merely `bymoving to the off position a master switch, or individual switchesprovided in each input.

Referring now more particularly to PGURE 2, the atomizing device isshown in more detail. The rotating bell-like atomizing member 16 is ofnylon, polyethylene or similar material of good high voltage insulatingcharacteristics. lt has an axial opening 16a, an adjacent portion 16hwhich flares sharply outwardly from the axial opening, and a rim portion1de extending forwardly more nearly axially with an inner surface about15 off from the axis, this portion being tapered to a relatively sharpforward atomizing edge 16d. The inner surface of the rotating member,from the axial opening clear to the atomizing edge, is covered with acoating 1de of a material which has high chemical resistance to theconstituents of the paint or other liquid coating material being used,high mechanical resistance to abrasion by the material owing thereover,and predetermined high elec trical resistance characteristics. Aspecific alkyd resin enamel-like coating material with finely `dividedcarbon particles therein has proved very satisfactory for this resistivelayer and is the subject of William D. Gauthier Patent No. 3,021,077,wherein it is more specifically described.

For purposes of this application, it is sutlicient to state r that thematerial should have suitable chemical and mechanical resistance, yandbe of high electrical resistance while still permitting a controlled owof very small currents therethrough. The resistance with which we arehere concerned is what may be termed a point-to-point resistance, inthat the factor of importance is the resistance to current ovv from agiven point on the surface to another surface'point, as at the atomizingedge. While the device illustrated achieves its desired resistancecharacteristics by using a body material for the bell which issubstantially completely non-conducting (as nylon) and a surface coatingwhich has some slight conductivity, it will be understood that therotating member could be formed of a single homogeneous materialprovided it had the desired pointtopoint resistance characteristics andsuitable mechanical strength. With a rotating bell member about 4 inchesin diameter, a suitable size for a hand atomizing device, it has beenfound that the resistance from the apex (at the axial opening) to therim or atomizing edge (when the same is placed in contact with a metalplate entirely around the extent of the rim edge) should be at least ofthe order of l0 megohms,`

and preferably of the order of 100 megohms. Under these circumstancesthe resistance per square (as for example per a one centimeter square)would be of the same order of magnitude as the above-mentionedapex-to-edge resistance, but it is more convenient commercially todetermine the resistance from the apex to the edge. lt will also beunderstood that if rotating atomizing devices of larger diameter areused, it may be desirable to materially `raise the resistance.

The bell member 16 is mounted on a rotating sha-ft assembly 29, as byrubber O-rings. This shaft assembly comprises a commercially availablehigh value resistor consisting of a hollow ceramic tube 29a of about l2inches in length, with an outer coating 2919 of high resistancematerial. A coatingon the right-hand tip of the ceramic tube provides anelectrical connection between the end of the resistance material 29h andthe inner or apex portion of the high resistance coating 16e on theinner surface of the bell member. The entire resistor is held within arigid carrier member 29C of phenolic `material or the like which rotateson bearing v surfaces provided by an outer insulating sheath 30,

which in turn lies within outer housing member 31 of polyethylene.

The outer sheath member 25 of the flexible conduit assembly 23 extendsinto the outer housing member 31, as may be seen at the left of thedrawing, the whole assembly being held in the hand atomizing device bythe cooperation of the cap 32 with an enlarged portion 33 molded in thesheath tube 25. The hollow flexible metallic shaft 2,4 within the tube25 is mechanically coupled, through a splined assembly 34, to therotating shaft assemblyV 29 carrying the bell member 16. Appropriateliquid seals in assembly 34 insure that all of the coating materialflowing from the end of paint tube 2'1 is transferred into hollow shaftassembly 29. Paint supplied through the tube 21 passes through thehollow shaft and spreads out on the inside of the bell during rotationthereof to continuously supply the atomizing edge with paint at a ratedetermined by the operation of the positive displacement pump, arepresentative rate of supply being l0() cc. per minute.

The circuit connection from the hot terminal of the power supplyiscompleted through the hollow flexible metal shaft (which is, however,sheathed from accidental Contact by the outer polyethylene tube), andany current reaching the atomizing' edge must pass through theresistance not only of the high resistance inner coating 16e on thebell, but also through the extended resistance layer '2911, this beinghere shown as of sufcient extent to prevent any possibility of arcingaround the outside thereof. It will be understood that suitableinsulation can prevent the arcing regardless of the actual extent of theresistor. In parallel with these fixed highV resistance values is acurrent path through the paint, from the point where it leaves theflexible metallic shaft end to the atomizing edge. While a very highconductive paint, in the embodiment here being described, would thusnullify the value of the high resistance means just described, theresistance of the paint column in a passageway of the size illustrated(which would normally be only 1,@ or i/l, of an inch in diameter) issuch that the resistance through the paint is at least several hundredmegohms and may be many thousands of megohms with the lacquers andsynthetic enamels normally used in commercial spray painting. It ispreferable to have the total effective resistance from the conductivesheath to the atomizing edge at least several megohms per kilovoltapplied by the high voltage source, wtih normal commercial operationspreferably having at least 300 megohms and normally 1000 megohmsresistance or more. Most commercially used coating materials provide ashunt resistance considerably higher than this, so that if the iixedresistances are arranged to provide a resistance at or about this value,the total working re. ,sistance will normally be approximately the samevalue.

Even if paint is quite non-conducting for all practical purposes, as isfrequently the case, the fixed resistances 9 will provide the necessaryvoltage at the edge of the bell for atomizing and charging the sprayparticles.

Again referring particularly to FIGURE l, the operator doing thespraying with the hand atomizing device would normally follow around themetal framework comprising the garnish molding, speaking in a verygeneral sense, and should normally keep the atomizing edge of the bellmember 6 or 8 inches away from the metal of the object, for example, ifa 100,000 volt pack is being used. However, hand operation or in factany operation wherein there are appreciable changes in air gap distancebetween the charging electrode and `an article, introduces a number ofproblems in a-n electrostatic coating system; Variation in gap distance,if the voltage between the electrode and the article remain the same,would obviously result in Very considerable variations in the averagepotential gradient. That is, if the bell and shaft were both of metalconnected to the 100,000 volt power pack, the average potential gradientat a l2- inch spacing would be only about 8000 volts per inch, whereasthe average potential gradient at a S-inch gap spacing would be 20,000volts per inch. An electrostatic coa-ting system of the kind justdescribed, with atomization being effected electrostatically, achievesvery` high paint deposition eliiciencies and is highly desirablecommercially. However, it has now been found that there are certain verydefinite optimum values of the average potential gradient, and thatthese do not remain the same regardless of spacing but vary inaccordance with the gap distance. A gradient at the film edge which istoo low does not atomize particles of the desired fineness for a qualitycoating; but improvement in atomization'increases with the gradient onlyup to a certain point, and thereafter deteriorates rather than furtherimproving.

We believe too high a voltage has certain field effects on the cuspscausing at least some of them to merge and A ive undesirably largeparticles, and in any event to flail or whip around in a manner whichalso results in much poorer uniformity in the size of the sprayparticles. Whatever the full explanations may be, however, we have foundthat there is a definite optimum average potential gradient forelectrostatic atomization, and that this does not remain constant withchanges in gap spacing. For example, with a 4-inch metal bell rotatingat about 1000 r.p.m. and supplied with 100 cc. per minute of a syntheticenamel, the optimum avera-ge potential gradient for electrostaticatomization at l2 inches is about 7500 volts per inch; at 9 inches isabout 9000 volts per inch; at 6 inches is about 11,700 volts per inch;and at 3 inches is about 17,000 volts per inch. We have found, as may bebest seen from reference to FIGURE 4, suitable selection of the controlresistance keeps the voltage at the atomizing edge of the bell in closeconformity with the optimum atomizing voltage throughout wide variationsin `gap distance, as from 3 inches to l2 inches. This results inmaintaining the quality of the finish on the article being sprayedthrough appreciable variations in gap distance, in a manner heretoforeimpossible.

Referring now more particularly to FIGURE 4, curves are shownillustrative of various conditions in electrostatic coating systems,both with and without the utilization of the present invention. Thecurve identified as 53 illustrates the gap break-down at various targetspacings, where the target is a rgrounded article of fiat sheet metal,and the atomizing bell has a 4-inch diameter atomizing edge with aradius of only about .005 inch, and connected to the negative terminalof a power pack having its positive terminal grounded. It will be seenthat the `gap breaks down a-t about 2 inches with 60 kilovolts, and at 3inches with 90 kilovolts. The curve identified as 59a illustrates thevoltage existing between the bell edge and the target at variousspacings with atomizers and power packs of the conventional typeheretofore used by the assignee of this `application wherein the onlyappreciable resistance is a l0 rnegohm limiting resistor built into thepower pack. With kilovolts applied to the bell edge at a l2-inch spacingfrom the target, the curve remains substantially flat as the spacing isreduced, with a very slight drop in voltage in the last inch or twobefore the break-down distance is reached, this being at about 25/sinches a-nd there still being almost 80 kilovolts at the bell edge.

The optimum .atomizing voltage for a 4inch bell as described above,being rotated at 900 r.p.m. and supplied with cc. per minute of aconventional commercial synthetic enamel, is indicated by the dottedline curve identied as 59h. In order to obtain optimum atomization in anelectrostatic system of the kind heretofore described, the voltageexisting across the gap between the atomizing edge and the target shouldfollow this curve 59h as closely as possible. It will be apparent thatthe curve 59a of the conventional system is at the optimum atomizingvoltage only at the l2-inch spacing and that material reductions in thisspacingresult in voltages greatly in excess of the optimum withattendant deterioration of the desired quality of atomization.

However, if the control resistance used in conjunction with a 100kilovolt pack is 1500 megohms, the resultant curve (identified as 59e)will be seen to follow the optimum atomization curve 59k in a verysatisfactory manner. That is, the curve 59e is never more than a veryfew percent away from the optimum atomization voltage curve 59h at anypoint, and intersects the gap breakdown line 58 when the spacing is lessthan l inch and when the voltage between the atomizing edge and thetarget has dropped below 30,000 volts. However, the curve is such thatit not only closely follows the optimum atomization curve S919 but alsoprovides in excess of 40,000 volts across the gap at all gap spacings of2 inches or more, so that satisfactory atomization and deposition takeplace at spacings from l2 inches down to as little as 2 inches. On theother hand, if the control resistance is of too high value, as forexample, 50,000 megohrns with a 10,0 kilovolt pack, the resultant curve(identified as 59d) will be seen to be so low at normal working spacingsillustrated on the chart that satisfactory atomization and deposition isnot obtained.

In order to achieve the desired automatic maintenance of theairgapvoltage drop at or near the optimum atomization voltage in anelectrostatic coating system of the character described and with presentcommercial power packs of 100 kv., the control impedance should bechosen to `absorb only a small percentage (as about 10%) of the appliedvoltage at the greatest gap space which is expected to be encountered innormal operation, but to` absorb at least as much voltage as the air gapresistance at the lesser air gap spacings which may be encountered inoperation. Again referring to the curve identified as 59e in FIGURE 4,it will be seen that the drop across the control resistance is about l5kilovolts at the l2-inc'n spacing, but is `approximately the same as thedrop across the air gap when this spacing has been reduced to 2% or 3inches. It will be understood that this will be true with a relativelysharp edged device of the character described, since the space currentincreases very materially as the gap spacing is reduced yand thusenables the control resistance to absorb the desired amount of voltagedrop thereacross even though the air gap resistance decreases to aneffective resistance of the same order as that of the controlresistance. Moreover, it will be apparent from the chart that when theatomizing edge approaches as close as l inch to the target or articlesurface, in the neighborhood of 70% of the voltage drop exists acrossthe control resistance with only the relatively small remainder existingat the air gap.

While higher source voltages and higher control impedance values arepreferable within certain limitations, the cost of a power packincreases much faster than the increase in potential available from it,so that it is at present commercially desirable to limit the maximumvoltage to be used to about 150 kilovolts. Under these conditions thecontrol resistance should be limited to the order of 15,000 megohms as amaximum in order to retain desirable atomization voltage variations andsatisfactory deposition. it is to be understood, however, that whereexpense is not an important factor, power packs can be built even todaywhich provide voltages about double those mentioned above as beingcommercially desirable; and that improvements in power supply technologymay well result in the ability to build even higher voltage packs forgeneral commercial use. However, our experiments have indicated thatwhile the control impedance should be at least several megohms perkilovolt, and preferably of the order of megohms per kilovolt orslightly more, it is undesirable to utilize more than about 100 megohmsper kilovolt; .and this relationship. obtains throughout all voltageranges which we believe likely to be used for electrostatic spraycoating.

Another important factor in the utilization of high voltages forelectrostatic spray coating is the matter of safety, both from thestandpoint of ignition of the combustible mixture which exists whenpaint solvents volatilize in air, and the matter of shock to theoperator. Certain solvents provide a more combustible mixture thanothers, one of the very commonly used paint solvents, xylene, providinga relatively quite ignitable mixture as it volatilizes into the airduring spray painting. We have found that even if the charging electrodeis brought up undesirably close to the object, to the point wheresparking occurs, as at a one-half inch spacing with control resistancesof the value mentioned above, this sparking is so weak, of such lowaverage intensity of energy level under conditions where there is a lowenough effective capacity beyond the control resistance, that even moreignitable mixtures than those normally encountered in a spray operationwill not be ignited. Moreover, while a low resistance of the order ofone-tenth of a megohm per applied kilovolt in systems of the type underdiseussion will prevent lethal shocks .and death of the operator if thecharging edge should be accidentally touched, suitable low effectivecapacities with control resistances of the values described results insuch a low shock level to the operator as not to even be distressing inthe event the charged edge should be accidentally touched or approachedtoo closely.

Even though .a control resistance of 1000 or 2000 megohms is used, wehave found that any high value of effective capacity beyond the controlresistance (i.e., between it and the charging electrode portion) willresult in a spark, upon close approach to an article or operator, whichwill ignite a highly ignitable mixture .and which will give such adistressing shock to the operator that few men will continue workingwith the system Where they are exposed to occasional accidental shocksof this type. We have found by experiment that with control resistancesof the values described the effective capacity ahead of such resistancemust be extremely low. A 4- inch metal bell member would have acapacitance several times the safe value or more, and well into the areawhere ignition and distressing shock would result even if a controlresistance of many thousands of megohms were used; and thus the presenceof a large object with its resultant geometric capacitance ahead of thecontrol resistance presented another problem from the standpoint ofsafety.

We have overcome this difficulty by the use of what might be termed veryhigh distributed resistance in the discharge electrode, so that a sparkoriginating at any one point ydoes not provide a discharge of anintensity greater than that which can be tolerated. That is, even thoughthe 4-inch bell member may have an actual capacitance (as measured to aplate spaced one-half inch from its charging electrode edge portion) ofabout 10 micro-l microfarads, the fact that the bell member has its mainbody portion of completely non-conducting material, with the specialcoating on the inner surface providing a very high point-to-pointresistance, prevents the surface charge at one point on the bell fromsubstantially instantaneously moving across to and being available fordischarge at another point which is brought too close to the object oroperator.

A conventional air spray gun or a conventional metal bell member of thetype heretofore used in electrostatic spray coating would in both caseshave such high capacitance and such good conduction of all the storedelectricity to a point Where a spark `originates that neither wouldbesafe, either as to ignition or distressing shock, even if controlresistances of the values heretofore considered were used between themand the high voltage source. By making any metal small enough, and bymak.-

ing a large body with a high geometric capacitance of such highdistributed resistance that the quantity of electricity stored thereinis not instantaneously available at a sparking point this danger isobviated. ln achieving effective capacity safety, it will be understoodthat it is the energy level at a sparking point which we have found tobe important, and that physically large devices which have truegeometric capacitance beyond those which could be tolerated can berendered safe in conjunction with our control resistances by having a.high distributed resistance therein or on the surface of' a relativelycompletely nonconducting body member. lt is also to be understood thatthere is some relationship between the value of the control Iresistanceand the amount of capacity which can be tolerated; but we have foundthat with control resistances of the character described, as, forexample, in the order of 1,000 rnegohms with kilovolts applied from thenegative terminal of a pack having its positive terminal grounded, theenergy level of a disruptive discharge, to va grounded polished metal spere of about one centimeter radius from an electrode ahead of thecontrol resistance, should preferably not exceed that from a polishedmetal sphere having a radius of about one centimeter replacing theelectrode in the same system, and in any event should not exceed thatfrom a sphere having a radius of about three centimeters.

We have found that a satisfactory and reproducible measurement of theamount of effective capacity which can ybe tolerated can be obtained byusing polished metal spheres of different diameters as the electrodebeyond the resistance. Such a sphere has a capacitance to an infiniteground around it which varies as its radius and which is about lmicromicrofarad per centimeter of radius. Moreover, the capacitance at aspacing of even a very few inches from. a grounded `object (such as ametal plate) is very close to the theoretical capacitance to an infiniteground. For example, a polished metal sphere having a radius of lcentimeter has a capacitance to innite ground of 1.1 micromicrofarad;and even when a flat grounded metal plate is brougnt within 1/i inch ofit, its capacitance as measured to the plate is only about 1.4micromiorofared. Accordingly, a polished metal sphere mounted on the endof the resistance element being used can be brought closer and closer toa grounded metal member (as the sphere of one centimeter radiusmentioned above) or to the end of the linger of an operator, until adisruptive discharge takes place, and the energy level of such dischargeis very reproducible with a igiven ball, resistor and applied voltage. For example, a polished steel sphere of 1 centimeter radius mounted onthe end of 1000 megohm resistor provides an unobjectionable discharge atany applied voltage below and up to 100 kv. The discharge from such acombination does not provide a disagreeable shock tothe person and hastoo low an energy level to ignite mixtures of the most readily ignitablenature, such as a saturated mixture of hexane and air, at 3 F. and atatmospheric pressure.

Higher resistor values (as for example 4000 megohms) lower appliedvoltage (as 50 kv.), and somewhat less ignitable solvents commonly usedin painting, as toluene or xylene, permit the effective capacity to beabout that of a sphere having a radius of 3 centimete-rs withoutobjectionable shock and without danger of ignition of the mostcombustible vapor-air mixture possible in the spraying zone, as asaturated mixture of toluene and airl at 62 F. or xylene and air at ll5F. It will be understood that the use of a polished metal sphere is forcomparative or test purposes only, and that such would not be used as acharging electrode in the normal electrostatic spray painting system.The energy level o-f the discharge from a polished metal sphere of l`centimeter radiusenergized through a 1000 niegohm resistor, however, isabout the same as that of a in-ch wire having a diameter .of .O50 inchsimilarly energized. Such a wire may be used Very satisfactorily as acharging electrode when atomization is separately effected, as will behereinafter more fully described, or may be considered analogous to theatomizing edge of a 4-inch diameter non-metallic bell with highlydistributed resistance, as heretofore described.

While the power pack provides a high voltage supply which iscommercially termed D.C., it wil-l be understood that this is not puredirect current in the absolute sense. In commercial practice the powerpack has its input connected to a conventional 60 cycle alternatingcurrent supply, and the high voltage output is rectified and partiallyfiltered to provide a unidirectional field-creating voltage; but thereis, of course, still an appreciable amount of ripple Accordingly, whilewe prefer to luse a pure resistance as the control means, any impedancewhich achieves the desired control action may be utilized. Also, whilethe control action in the embodi ment just described is a result of thetotal effective resistance provided by two parallel resistances (onebeing the resistor and the other being the resistance of the paintcolumn), it is to be understood that the desired control impedance canbe achieved in a number of ways. For example, it may be achieved by theuse of carbon, metallic oxide, or other commercial resistors; byequipment components made of or `coated with materials having thedesired resistance characteristics; by the resistance of the paintcolumn or film; by the inter-element impedance of a high voltage vacuumtube; or by combinations of the above or other suitable impedances.

Referring now more particularly to the embodiment of the invention shownin FIGURE 5, a modified form of a hand-held atomizing device isidentified in general as 40. In this device three separate connectionsare made to the rear of the device, although for convenience the three separate conduits 41, 42 and 43 are brought together into a singleflexible conduit assembly preferably covered with a grounded flexiblemetallic outer sheath 44. The bell member 45 is mounted upon a rotatableshaft 46 connected to a flexible drive shaft in the conduit 42. In thisembodiment of the invention, the bell member 45 is again ofnon-conducting material, such as nylon, with a surface layer 47 ofpredetermined resistances; but in this case the resistance layer 47 ison the exterior of the bell member while paint flows on the innersurface, contact between the paint and resistive layer occurring only atthe atomizing edge 45a. High voltage is brought in through theelectrical conduit or 1ead.41, passes through two fixed resistors 48 and49 which may be, by example, of 500 megohms each, with the two providinga total extent of about l2 inches. The forward end of resistor 49 iselectrically connected to a very small metal leaf spring 50` which makesrubbing contact with a portion of the resistive coating 47 to transfercurrent thereto.

This embodiment of the invention includes a liquid control valve in theatomizing device itself, the actuating member for the valve being thetrigger element 51 pivoted at 52 on a handle 53. When the parts are inthe position illustrated a valve member 54 closes off the liquid supplyconduit 43, being urged to closed position by a spring 55. When thelower end of the trigger element 51 is pulled toward the handle 53,however, its engagement with the Valve actuating member 56 lifts thevalve 54 off its seat, and paint can then flow through the passageway 57to the hollow shaft 46, and thus through the axial opening in the bellmember to spread out in a film on the inner surface thereof and move tothe atomizing edge as a result of the rotation of the bell.

In this embodiment of the invention the flexible shaft and the paintsupply conduit may be grounded, since they are isolated from the highvoltage electrical supply, and thus the rotating motor and the paintsupply means may be merely set on the floor or be otherwise suitablysupported, but without the need of any high voltage insulation.Moreover, since there is a control valve in the atomizing device, nopump need be used for the paint, but instead the paint can be suppliedif desired from a simple pressure container (generally termed a pressurepot) having air under pressure (as at 30 pounds per square inch) overthe paint. Moreover, the handle and trigger can be of metal and groundedby being electrically connected to the grounded metal sheath 44. Thisresults in a situation where any failure of insulation could not resultin shock to the operator.

While this embodiment has a series or control resistance analogous tothat of the first described embodiment, it is to be noted that in thisform in FIGURE 5 the resistance of the paint column extending from thevalve chamber to the atomizing edge is in parallel with the air gapresistance between the edge of the bell and the article being coated,rather than being in parallel with the resistors 48 and 49. This resultsfrom the fact that the paint column is grounded at the valve portion ofthe device, so that it is in effect connected to the ,same terminal ofthe power pack as the article, the bell edge potential and the averagepotential gradient of the field, duringvariations in gap spacing betweenthe bell and the article being thus controlled in part by the seriesresistors 4S and 49, and in part by the parallel impedance provided bythe paint column. Where the paint has a resistance characteristic suchthat passageway diameters and lengths will fall within reasonable andpractical values to achieve desired resistances, better control of thecharging electrode portion voltage may under some circumstances beobtained. It is also to be understood that if the paint is keptcompletely insulated from ground and at high potential in this form, oris of such high resistance as to be substantially completelynon-conductive, similarl control may be obtained by utilization ofanother impedance between the charging electrode (here the edge portionof the bell) and ground, in conjunction with the series resistanceprovided by the resistors 48 and 49.

In connection with the electrostatic atomization effected in both of theembodiments heretofore described, we have found that the presence of thehigh series resistance improves atomization by maintaining the atomizingedge at or near optimum atomizing voltage as the gap spacing is varied.It has heretofore been thought that at least one fifth of a milliampereof current was necessary for suitable electrostatic atomization, but Wehave found that we can not only retain good atomization and charging ofthe spray particles, but even improve these, with currents of the orderof onetwentieth of this, or even less, as about l0 microamperes. v Thisdiscovery has .enabled us to use resistances of several thousand megohmsin conjunction with the now generally commercially used power pack ofkv.

Referring now more particularly to FIGURE 6, an embodiment isillustrated which retains the advantages of deposition efficiency andsafetyfof the forms heretofore described, but which effects atomizationby hydrostatic pressure, rather than electrostatically. The atomizingdevice shown in this figure is again of a type adapted to be held in thehand, being generally pistol-like with a grounded metal handle portion60 and a barrel-like portion 6l about l0 inchesl in length made ofnon-conducting material suitable for high voltage purposes, aspolyethylene. Paint is supplied through an insulating high pressure hose62 made of a material such as nylon leading from a high pressure vesselnot shown.

Paint is delivered through hose 62 to a hydrostatic atomizing nozzle 63.ln order to minimize wear as the paint passes through the very smalloriiice of the nozzle, the nozzle should be made of some suitable wearresistant material such as a sapphire, stainless steel, or tungstencarbide. lf the nozzle is of conducting material such as stainlesssteel, it must be kept small enough to avoid an objectionable dischargebecause of its capacitance. That is, it should be small enough that theenergy level of a discharge therefrom preferably should not exceed thatwhich would be available, under the test conditions previouslydescribed, from a metal sphere of about one centimeter radius, nor inany event from a sphere of about 3 centimeters radius. Nozzle 63 issealed against the end of hose 62 and supported in barrel 61 by apacking gland assembly `64 comprising a solvent resistant packing 64awhich is compressed against the tube and nozzle by threaded nylonmembers 64b and 64C.

In order to secure pressure atomization of a neness suitable for qualitycoatings, the paint in this case would be subjected to a pressure ofseveral hundred pounds or more per square inch in the pressure Vessel orsupply chamber. The outlet orifice of nozzle 63 would preferably beelongated in cross-section, with its narrowest dimension only a fewthousandths of an inch7 and its longer dimension 2 or 3 times as great.Such an atomizing nozzle discharges the paint into a fan-like iilm ofsuitable characteristics to achieve the desired atomization. in theembodiment illustrated, the paint supply would be be mounted on aninsulated table or the like, as in the embodiment illustrated in FiGUREl, and would have the hot terminal of the high voltage connectedthereto.

To provide the necessary insulation for hose 62 carrying paint at highvoltage, an insulating conduit 61a extends from the insulating barrel 61to the paint supply. Members 6l and 61a may be fabricated integrally,or, if welded together as shown, the weld must be capable of providinginsulation between the high voltage paint hose 62 and grounded handle60. A grounded sheath 60a such as a braided metal may be provided overconduit 61a to insure all exposed surfaces of the gun up to barrel 61are at ground.

The paint column itself in hose 62 provides the high resistanceconductor for the high voltage applied to nozzle 63. The controlimpedance provided by the resistance of the paint column not onlyprovides safety against objectionable discharges but also achieves thehighly advantageous control action on average potential gradientvariation during gap variations in accordance with our invention aspreviously described. Coating materials and their solvents possesswidely Varying degrees of electrical conductivity and inorder to controlthe total resistance of the paint column within tube 62 a metal wire 65may be inserted in tube 62l extending from the high pressure supply tosuch point within tube 62 such as will give the length of paint columnrequired to provide the desired sole control resistance between the highvoltage source and nozzle 63.

The supply of paint to the nozzle may be controlled by a trigger 66mounted on the handle. In the embodiment shown, operation of trigger 66depresses a spring 67 and makes electrical contact between the terminalsor" a switch 68 which through wires ,`69 control the operation of avalve controlling the supply of paint through tube '62. This valve ispreferably of the three-way type which provides a connection between thehigh pressure vessel and nozzle 63 when contact is made in switch 68 andwhich liti vents tube 62 to the atmosphere when the contact is broken.

important control and safety aspects of this invention can also beembodied in systems wherein the depositing eld extends from the articleto an opposite electrode which is not a part of lthe atomizing device,and a representative system of this type is shown Ain FIGURES 7 and 8.Four small electrode wires 71a-d are arranged horizontally in a singleplane with their ends one foot apart at what may be considered thecorners of a square, as may best be seen in FIG-URE 8. These fieldelectrodes are supplied with current through control resistance means72a-d, these resistances having values of the character heretoforedescribed and being of sufficient length that there is no danger ofarcing around from the electrode wire 71 to the lead 73 from the highvoltage supply pack 74. A conventional air spray gun 75 is illustratedas mounted on a support pedestal 76, the gun and pedestal being atground potential and uninsulated, although it is to be understood thatthe spray gun can be hand-held and freely movable. Paint from a supplytank 77 is delivered through the hose 78 when compressed air is suppliedto the gun through the hose 79. The axis of the spray gun intersects thecenter of the square formed by the tips of the electrode wire 71, but toone side of the plane of the electrode wires, as for example, 6 inchestherefrom. On the opposite side of the plane of the electrodes is anarticle 80 here illustrated as a breadbox, carried by a conveyor spindle81 with rotating means including the wheel element S2 bearing against arotating rail 83.

It will be understood that a succession of such articles are movedthrough the coating zone by the conveyor, and their rotation during thecoating causes the corners to be much closer to the electrode wires thanthe center of the wall panels. It is also to be understood that aslightly bent or loose spindle or an article improperly positioned onthe spindle would, of course, result in even greater variations in thedistance between the spray charging electrodes 7l and the article beingcoated. In the past this has been taken care of from a safety standpointby utilizing a much greater electrodeto-article spacing than would be`desirable from the standpoint of deposition e'iciency. Where theclosest approach of an article portion anticipated to occur in normaloperation is double the sparking distance for the voltage used, it isobvious that the average potential gradient Will be at about the minimumof the desirable working range when such closest portion is presented;and that when the center portions of the walls of the bread-boxillustrated are being presented toward the spray gun, or if the articlewobbles somewhat further away as a result of a bent spindle, the fieldgradients will be substantially below the desired value. This results inthe Vair blast blowing away more spray particles than would be the caseif the average field gradient were at all times at a desirably highlevel. While electrostatic effects have advantage with average potentialgradients of about 5,000 volts per inch at the six to twelve inchspacings being discussed, average gradients of well over 10,000 voltsper inch are desirable for maximum deposition eiciencies in Va system ofthis character.

`ln the system illustrated, the side walls of the breadbox, whenpresented directly toward the spray gun by a straight spindle having thearticle properly positioned thereon, may be located only 6 inches fromthe electrode wires 71 although a 100 kv. power pack is being used, Witha resistor 72 absorbing about 10% of the voltage, about 90,000 voltswould exist on the electrode wires 71 and the average potential gradientof the field to the article under the conditions just described wouldthus be about 15,000 volts per inch. Even if a bent spindle or anyimproper positioning or swaying of the article results in the distanceincreasing a couple of inches, the average potential gradient is stillin excess of 10,000 volts per inch. Yet as the article rotates andpresents a corner directly toward the spray gun, or if some other factor17 causes it to corne closer, reduction in the gap distance between theelectrode Wires and the closest article portion (as for example to 3inches) still permits an operative and highly efiicient field to exist,the resistor absorbing sufficient voltage drop in the system to preventany disruptive discharge down to a very close spacing, as one-'half inchor one inch, and prevent it from being objectionable even then.

We have found it desirable to have the electrodes of appreciable length,extending preferably at least several inches from the resistors, a verysatisfactory embodiment of this invention having lbeen constructed witheach electrode wire being 6 inches long. The highly ionizing effect of asmall wire (as ten thousandths of an inch in diameter) repels the paintand not only charges lthe spray but also creates conditions which keepthe spray from painting back onto the remotely located sheaths or coversof the resistance elements, which is unsatisfactory. Moreover, the sharpconfiguration of such a ne wire, with attendant higher space currentsacross the air gap than would occur from a polished sphere or otherblunt electrode, has a desirable action in connection with the controleffect of the resistor, and in minimizing the elective capacity of agiven electrode size. The conditions as to capacitance must still beobserved, however, and the size of the metal electrode should be keptsmall enough that the discharge therefrom should preferably not exceedthat from a metal sphere of one centimeter radius, or in any eventshould not exceed that from a metal sphere of three centimeters radius.

in :addition to using electrode wires having a length of at leastseveral inches, but not exceeding a suitable capacitance, we find thatwhere a plurality of electrode wires are used and supplied with currentthrough independent control impedances, the electrodes should be spacedat least several inches apart. If the electrodes are too close togetheran operator or article can touch several of them at once and this wouldresult in an objectionable discharge since the capacities and impedanceswould all be effectively in parallel under such conditions. With the useof a system of the character embodying the above discussed factors,however, safety consider-ations are retained while achieving highaverage field intensities despite appreciable vari-ations in gapdistance, with resultant improved deposition eiiiciency. in a systemconstructed in accordance with the Idisclosure of FIGURES 7 and 8, forexample, used in the spray coating of met-al broom handles 1 inch indiameter and spaced on 3 inch centers, deposition efficiency with such-a system was about 10% better than with the same `electrode and spr-aygun arrangement without the control impedances `and with the path ofarticle movement then being l-2 inches from the electrode rather than 6inches in View of the necessity of being double the possible arcingdistance.

In a system utilizing charging electrodes separate from the atomizingdevice, as illustrated in FIGURES 7 and 8, laverage gradients which arein excess of 10,000 volts per inch, and preferably 15,000 volts per inchor more, are desirable for maximum `deposition eiiiciency at spacings ofsix to twelve inches, for example. In the absence of a controlresistance the minimum gap space between the electrode and :articlewould normally be required to :be about 12 inches with la 100 kv. pack,with the average potential gradient then being a little less than 10,000volts per inch and dropping rapidly as the gap is increased by rotationof the article, or swaying of the Iarticle away from the electrode. Wehave found that very much improved deposition efficiencies can beachieved, particularly upon articles presenting a substantial amount ofopen space relative to the metal surface lbeing coated, by not onlyutilizing control resistance means but also by materially raising thepower pack voltage beyond that which would otherwise normally be used,and have obtained very desirable results by use of a 160 kv. power packwith .appropriate control resistances. For example, if a 5,000 megohmcontrol resistance is used with a 160 kv. power pack, the spacingarrangement can be such that if the minimum spacing encountered is 4inches, the average potential gradient at that spacing would be about18,000 volts; and yet at a l2-inch spacing the gradient would still beabout 10,000 volts per inch. This is less than a 2. to 1 change ofaverage gradient for a 3 to l change in gap spacing; and enables theaverage gradient to normally be at or about 15,000 volt per inch, withthe gradient at the large l2-inch spacing still being about 10,000 voltsper inch.

In general, and speaking of all of the embodiments of our inventionheretofore described, it will be understood that it is preferable toutilize both a suitable control impedance and a low effective electrodecapacity. The geometry and surface area of atomizing devices of thecharacter heretofore generally used commercially may provide yanobjectionable disruptive discharge even when suitable control impedancesareused in series therewith. We have found that large area atomizingdevices, such as latomizing bells or hand guns effecting mechanicalatomization can have their effective capacities reduced fto Ia suitablevalue by being made substantially entirely of non-conductive materialwith a high distributed resistance. Where it is desirable to use a metalelectrode, making it not only of small capacitance and preferablyrelatively sharp, as by using a wire electrode of small diameter,achieves the desired low effective capacity. Moreover the automaticcontro-l action of the impedance element as shown is completely reliablein operation, without any complex electronic circuits or equipmentsubject to failure in operation.

The optimum value of the control resistors varies with the amount ofactual or effective impedance present in the remainder of theelectrostatic system, and particularly in the power supply. Even thoughthe actual specific or ohmic value of the resistance in the power supplymay be relatively small, the power supply can provide a large effectiveportion of the impedance in the system by virtue of its voltageregulation characteristics. Accordingly, it will be understood that theohmic value of the resistors illustrated and heretofore described asexamples of our invention will desirab-ly be made larger or smallerdepending upon the total effective impedance of the electrostaticsystem, including the effective impedance ofthe power supply itself,being smaller when the effective impedance of the power supply islarger.

Referring now more particularly to the embodiment of our inventionillustrated in FIGURES 9 and l0, we

again utilize a control impedance system in connection with atomizingand charging means of low effective capacity. In this form of ourinvention the effective system impedance still is very high, with asubstantial amount of resistance still immediately adjacent the chargingelectrode, but with the power supply providing an effective portion ofthe system impedance.

Referring first to the schematic diagram shown in FIGURE 9, theatomizing and charging device 1011 is represented as a rotating bellhaving a very high resistance to reduce its effective capacity a-t leastto below that of a sphere of polished metal of 3 cm. radius, andpreferably of only l cm. radius. This atomizing and charging device mayin all respects be analogous to the device 16 illustrated in FIGURES 1and 2, and therefore it will not be more fully described here. Theatomizer 101 is `adapted to be suitably spaced from each of a successionof articles conveyed through the coating zone, one such grounded articlebeing identified here as 102. A. particle charging and depositingelectrostatic field is created between the atolnizer 101 and the article102 by the utilization of a power supply and suitable connectionsincluding a high resistance 103 of substantial value immediatelyadjacent and connected to the atomizer 101.

The various modifications of our invention illustrated in FIGURES l to 8and heretofore described in this specification have been described asenergized by a conansa/tee ventional commercial power pack of goedquality, a type of pack which has good regulation characteristics andmaintains its output voltage close to its rated value despitesubstantial increases in load current. Moreover, all practicalcommercial power supplies known to be used lin electrostatic spraycoating before the invention illustrated herein in FIGURES 9 and l()maintain sufficient volta-ge regulation (even Ithough relatively poor),that they were not sufiiciently self-regulating for optimumelectrostatic atomization, or fully safe against fire or shock hazard,in Ithe absence of additional high resistance of the order of theminimums heretofore described in this application, even Iwhen utilizingvery low effective capacity of the charging means cooperating with thearticle to produce the particle charging and depositing electrostaticfield. The power supply shown herein, however, produces suicienteffective system impedance, sufiicient drop of output voltage uponincrease `of load current between the charging electrode and the articlebeing coated, that the resistance external to the pack can be lower thanthat used in connection with a well regulated voltage supply, and can beapproximately the same in megohms, or at least of the same order as,-t-he kilovolts being ap*- plied to the charging electrode at any giveninstantaneous operating condition.

While a power packof the ladder type has been known to us, we have knownthat such a pack will not in and of itself, without the substantialcontrol impedances of the character heretofore discussed, effectadequate control of voltage during variations of spacing between theparticle charging means and the article. However, we have found that ifsuch a power pack is associated 4with an input network of a particulartype as hereinafter more fully described, that satisfactorily effective`impedances for the coating system can be provided by the use of aresistance externalfto the pack, and preferably closely adjacent to thecharging electrode, of a rvalue of at least about l megohm per kilovoltof potential applied to the field creating electrode at the lowest rangeof spacings expected during normal operation, and in any `event at least40 megohms; provided, of course, that the charging electrode (theatomizing bell as illustrated in FIG- URE 9) has a low effectivecapacity of the small values called for throughout this application.

The ladder-type power pack identified in general as 104e: is suppliedwith alternating current through an input network identified in generalas `ttt-tb, a network which materially increases the Vreduction of D.C.output voltage upon increases in load current. In fact, in oneparticular power supply of this character which we have built, theladder type power pack itself, when supplied with an input of 110 volt,60` -cycle alternating current (but without any input network), providedan output voltage of about 95 kilovolts at 20 microamperes of outputcurrent, 60` kilovolts at 0 microamperes, 53 kilovolts at 60microamperes, 48 kilovolts at 70 microamperes, and 45 kilovolts at 80microamperes. It will be noted that while there was an initialsubstantial drop in voltage upon an increase of output or load currentfrom to 50 microamperes, that thereafter the voltage reduction decreasedmarkedly and the curve of voltage reduction flattened out appreciably(and the voltage remained at a relatively high value) at output currentsindicating a relatively close approach of the charging electrode to thearticle being coated. On the other hand, the same power pack whensupplied through an input network of the character illustrated here,with 'an input voltage of about 110 volts of 60 cycle A.C., ex-

hibited a markedly greater reduction in output voltage at the higherlevels of output current. A-t 20 microamperes output current (fullysatisfactory for electrostatic atomization) the output voltage -w'as 85kilovolts, which would be satisfactory for article-electrode spacings of8 or 10 inches; at 50 microamperes, however, the output voltage was only43 kilovolts; at 60 microamperes it was 30 kilovolts; at 70 microamperesthe output voltage was 23 kilo- 2t) volts; and at microamperes it was 18kilovolts. Moreover, tby the time the output current had reached 84microamperes, in the power supply now being described, it dropped inoutput voltage to l0 kilovolts, whereas the ladder network alone(without the input network) was still 43 kilovolts.

It will -thus be apparent that a large portion of the effective systemimpedance (the electrical changes in voltage across the air gapwhichtake place upon variations in the gap spacings or distance) canthus reside to a substantial extent in the power supply and need besupplemented only by a high resistance between it and the chargingelectrode, preferably near the electrode to minimize the effect ofcapacity present in the connecting lead or cable, which has a value inmegohms approximately equal to the kilovolts applied to thefield-creating electrode at the lower of the spacings expected to beencountered in normal operation, as 3 or 4 inches. The total systemimpedance, including this high resistance external to the power supply,under such circumstances is suflicient to provide optimum electrostaticatomizing voltages during variations in spacings between theelectrostatic atomizer and the article being coated, and safety fromfire and shock under any conditions of electrostatic coating when thisimpedance is combined with adequately low effective electrode capacity.

To describe the circuit in more detail, it will be seen that one leg orside of the ladder power supply network comprises (in the circuitillustrated in FIGURE l0) four capacities ltSa and lliSd, inclusive,connected in series between ground and the direct current outputterminal. The other leg of the ladder network similarly comprises fourcapacities connected in series, here identified as 166:1 to 106d,inclusive, the capacities in this case being connected between oneterminal of a transformer secondary 107b (having its other terminalconnected to the other or first described leg of the ladder network),and the other end being connected to rectier 168k. Intermediate the twolegs are a plurality of alternately oppositely connected rectifiersltlSa to 198k, inclusive, these rectiiiers and their connections formingwhat are sometimes termed the crossconnections between the legs of thelatter network. In operation of a network of this type, when a voltageis first developed across transformer secondary 1071) it results incharging condenser 106a. When the polarity reverses in the transformersecondary voltage impressed upon condenser a by the secondary issupplemented by voltage and current delivered from the condenser 10611through rectifier 10811. As the process continues up the ladder, anoutput voltage is developed which is many times multiplied over themaximum A.C. voltage developed in the transformer secondary ltl'lb.However, as mentioned heretofore, such a network is not in and of itselfsuiiicient to provide proper control to achieve optimum electrostaticatomizing voltage during variations `in spacing from an article beingcoated, nor -to provide adequate safety against fire and shock hazard.

As mentioned hereinabove, we have found that more I of the effectiveimpedance of the entire coating systems circuit can be provided 4in thepower supply by feeding the input of the ladder network from a reactivebridge, as illustrated in the lower part of FIGURE 10. This bridge inputnetwork has two opposite corners 10961 and 109b supplied with thealternating current input, as conventional 60 cycle 11() volt power; andthe other two corners of the bridge, here identified as l10n and 110b,are connected to and provide voltage to the primary 107er to energizethe utilized capacitors 112e and 11211 of a 6.75 mfd. capacity; and asinductances used coils which had a D.C. resistance of 90 ohms and aninductance which varied from about 9 henries with a 115 volt input andno load on the output of the bridgenetwork to about 35 henries at shortcircuit on the output. In the particular power supply we are describingthis coil was provided by winding 1160 turns of No. 34 wire in twelveequal l-ayers on the center leg of a closed (El) core with a 3A stack of5/8 wide laminations of good grade of magnetic iron, sometimes termeddynamo grade.

We have found that it is not desirable to have a balanced bridge for theinput network. Instead, we prefer a bridge relationship wherein theoutput voltage does not drop markedly until a predetermined current hasbeen reached and then drops off as rapidly as possible thereafter. Wehave found that this is best effected by using capacitors with areactance much less than that of the other components of the bridge, inthe case given above the capacitors having a reactance of only about//l@ of that ofthe inductances at no output load and with a 60 cycleinput.

It will be understood that a reactive input system for energizing theladder-type power pack can be provided by suitable arrangements otherthan the capacitance-inductance bridge illustrated and just described.For example, a limited current transformer of the type illustrated inFIGURE 3 of Sola Patent 2,212,198 (with a characteristic curve of outputvoltage relative to load current as illustrated in FIGURE 7 of thatpatent) is very satisfactory when suitably designated for the loadcurrents ydesired in the electrostatic coating system utilizing. theoutput of the ladder-type pack. Y

In spite of the very greatly increased voltage dropping characteristicof such a power supply upon reduction in spacing between the electrodeand article and attendant increase in output current, we have found thatsatisfactory operating conditions cannot be provided by such a powersupply alone. We find it of extreme importance that the electrode systembe made of very low effective capacity, in any event as low as thatwhich would be provided by a polished steel ball of 3 centimeter radiusat the same point in the system, and preferably not above the effectivecapacity which would be provided by a polished steel ball of lcentimeter radius; and that the system include a high resistance in theconnection between the power pack output and the charging orfield-creating electrode, a resistance of at least 40 megohms. While theimpedance throughout the entire field-creating circuit effective uponthe voltage across the gap between the electrode and article at low'spacings is of considerable importance, we v find that use of anelectrode system of low effective capacity, and an associated resistorin the circuit having a high resistance value and preferably closelyadjacent the charging electrode, are of absolute necessity in achievingthe operation desired by us.

While we have shown and described certain embodiments of our invention,it is to be understood that it is capable of many modifications.Changes, therefore, in the construction and arrangement may -be madewithout departing from the spirit and scope of the invention asdisclosed in the appended claims.

We claim:

1. Apparatus for spray coating a plurality of articles, comprising anatomizing device adapted to be held in the hand and having at one endthereof a rotating member having an atomizing and ionizing chargingportion with an extent of at least several inches, said membercomprising high resistance material and the physical dimensions andresistivity of the member being such as to make it of low effectivecapacity, not greater than that of a metal sphere of a radius of aboutthree centimeters; conveyor means for translating articles substantiallyat ground potential successively along a predetermined path through acoating zone where atomized particles are deposited thereon, thedistance between said atomizing device and the articles being subject tovariation which could in a conventional electrostatic spray coatingsystemcause an objectionable change in potential gradient; voltagesupply means for creating a high unidirectional potential difference ofat least forty thousand volts and of the order of at least five thousandvolts per inch average gradient between said charging portion and theclosest article portion to create a depositing field, the connectionbetween the voltage supply means and said charging portion beingcompleted throu-gh a high resistance immediately adjacent said chargingportion of the electrode and including said high resistance material,and having a resistance of at least the order of several hundred megohmsand at least several megohms per kilo- Volt of the voltage source, butnot in excess of about one hundred megohms per kilovolt, whereby changesin average potential gradient resulting from variations in the distancebetween the articles and atomizing device is materially reduced andobjectionable discharges are pre- Vented; a handle portion on theatomizing device beyond arcing distance from said charging portion; andmeans for supplying a controlled iow of liquid coat-ing material havingsome conductivity but relatively high resistance to said atomizingdevice for spreading out in a thin annular film on said rotating memberfor electrostatic atomization therefrom into the depositing field fordeposition of spray particles on the article while still in liquidstate, the voltage supplied from said source being substantially inexcess of the voltage -which would be optimum for said electrostaticatomization in relation to the distance between the atomizing portionand an article at at least the lesser distances in the absence of saidhigh resistance and its effect on changes in the average potentialgradient.

2. Apparatus of the character claimed in claim l, wherein the resistanceof said liquid coating material is electrically in parallel 'with saidhigh resistance.

3. Apparatus of the character claimed in claim l, wherein s-aidatomizing device includes flow control means having a control elementadjacent said handle portion, said control element and iiow controlmeans being grounded whereby the resistance of said liquid coatingmaterial is electrically in parallel with the space between theatomizing portion and the grounded article being coated.

4. Apparatus for spray coating an article, comprising an atomizingdevice having an atomizing zone adjacent one end thereof adapted to beheld in the hand and comprised substantially of non-conductive material,Said device -having -a very low effective capacity, not greater thanthat iof a metal sphere of a radius of about three centimeters, thedistance between said device and the article being subject to variationswhich could in a conventional electrostatic spray coating system causean objectionable change in potential gradient; voltage supply means forcreating a high unidirectional potential difference of at least fortythousand volts` and at least sever-al thousand volts per inch averagegradient between said atomizing device and the closest article portionto create a depositing eld, the connection between the voltage supplymeans and the atomizing zone of said device being completed through highresistance means having at least a substantial portion thereofimmediately adjacent said zone yand having a resistance of at leastseveral megohms per kilovolt of the voltage source; and means forsupplying a controlled flow of liquid coating material to said atomizingdevice for atomization therefrom into `the depositing field fordeposition of spray particles on the article while still in liquidstate, the voltage supplied from said source being in excess of thevoltage which could be used at at least the lesser distances between theatomizing device and article in the absence of said high resistancemeans and its effect on changes in the average potential gradient.

5. Apparatus of l the character claimed in claim 4, wherein saidatomizing device has a handle portion beyond arcing distance from theatomizing zone, and wherein atomization is effected by fluid pressure.

6. Apparatus for spray coating a plurality of articles,

comprising charging electrode means having a plurality of separateionizing portions several inches apart and each with an appreciableextent and each having an effective capacity not greater than that of ametal sphere of a radius of about one centimeter; conveyor means fortranslating conducting articles at ground potential successively along apredetermined path passing adjacent but at least several inches fromsaid charging electrode means, the distance between said electrode meansand the articles being subject to variations which could in aconventional electrostatic spray coating system cause an objectionablechange in potential gradient; voltage supply means for creating `a highunidirectional potential difference of at least forty ,thousand voltsand at least several thousand volts per inch average gradient betweensaid ionizing portions and the closest article portion to create adepositing held, the connection between the voltage supply means andeach said ionizing portion being separately completed through highresistance means immediately adjacent each said charging portion andhaving a resistance of at least several megohms per kilovolt of thevoltage source; and means for supplying a controlled flow of liquidcoating material and for atomizing it into the depositing ield fordeposition of spray particles on the article while still in liquidstate, the voltage supplied from said source being in excess of thevoltage which could be used in the absence of said high resistance meansand its effect on changes in the average potential gradient.

7. Apparatus for electrostatically depositing liquid coating material onan article, comprising a source of unidirectional high potential of atleast forty thousand volts, an ionizing charging electrode electricallyconnected to said source, means for supporting the article to be coatedsubstantially at ground potential, means connecting said article incircuit with said high potential source to establish an electrostaticiield between said charging electrode and said article, a source ofliquid coating material, and means for atomizing said `liquid coatingmaterial into said electrostatic held, the distance between saidcharging electrode and the article being subject to variations whichcould in a conventional electrostatic spray coating system cause anobjectionable change in potential gradient, said charging electrodebeing composed o such material and so dimensioned that its effectivecapacity is not greater than that of a metal sphere of a radius of aboutthree centimeters, said means connecting said current source to saidcharging electrode including resistance means having at least asubstantial portion thereof immediately adjacent the charging electrodeand of the order of ten megohms per kilovolt applied by said potentialsource to minimize variations in the potential gradient of saidelectrostatic field that would otherwise occur as a result of variationsin said distance, whereby at all distances objectionable discharge ofelectrical energy of a character to initiate a re or shock to theoperator is avoided.

8. Apparatus for electrostatically depositing liquid coating material onan article, comprising a source of unidirectional high potential; anionizing charging electrode electrically connected to said source; meansfor supporting the article to be coated; means connecting said articlein circuit with said high potential source to establish an electrostaticheld between said charging electrode and said article; a source ofliquid coating material; and means for atomizing said liquid coatingmaterial into said electrostatic field, Ithe distance between saidcharging electrode and the article being subject to variations whichcould in a conventional electrostatic spray coating system cause anobjectionable change in potential gradient, said charging electrodebeing composed of such material and so dimensioned that its effectivecapacity is not greater than that of a metal sphere of a radius of aboutone centimeter, said means connecting said high potential source to saidcharging electrode including resistance means having at least asubstantial portion thereof immediately adjacent the charging electrodeand of magnitude sutlicient to minimize variations in the potentialgradient of said electrostatic iield that would otherwise occur as aresult of variations in said distance, whereby at all distancesobjectionable discharge o electrical energy of a character to initiate atire or shock to the operator is avoided.

9. Apparatus for electrostatically depositing liquid coating material onan article, comprising a source of unidirectional high potential; acharging electrode electrically connected to said source; means forsupporting the article to be coated; means connecting said article incircuit with said high potential source to establish an electrostaticfield between said charging electrode and said article; a source ofliquid coating material; and means for atomizing said liquid coatingmaterial into said electrostatic eld, the distance between said chargingelectrode and the article being subject to variations which could in aconventional electrostatic spray coating system cause an objectionablechange in potential gradient,`

said means connecting said high potential source to said chargingelectrode including resistance means immediately adjacent the chargingelectrode, the physical dimensions and electrical conductivity of thecharging electrode, the total electrical resistance of said circuitbetween any discharge point on said charging electrode and said sourceof high potential and the magnitude of said high potential being suchthat for any distance, the total flow of current between the electrodeand article, resulting from the potential applied to said circuit andthe discharge of the capacitance of said charging electrode, has -avalue and duration less than that which will initiate a disruptivedischarge of an energy level so high as to be objectionable.

l0. Apparatus for lelectrostatically depositing liquid coating materialon an article, comprising a source of unidirectional high potential; acharging electrode electrically connected to said source; means forsupporting the article to be coated; means connecting said article incircuit -with said high potential source to establish an electrostaticfield between said charging electrode and said article; a source ofliquid coating material; and means for atomizing said liquid coatingmaterial into said electrostatic field, the distance between saidcharging electrode and the article being subject to variations whichcould lin a conventional electrostatic spray coating system cause anobjectionable change in potential gradient, said means connecting saidcurrent source to said charging electrode including resistance meansimmediately adjacent the charging electrode and of at least severalmegohms per kilovolt of the high potential source, the physicaldimensions and electrical conductivity of the charging electrode beingsuch that the effective capacity is not greater than that of a metalsphere of a radius of about one centimeter, the total electricalresistance of said circuit between any discharge point on said chargingelectrode and said source of high potential and the magnitude of saidpotential being such that for any distance, the total flow of currentbetween the electrode and article, resulting from the potential appliedto said circuit and the discharge of the capacitance of said chargingelectrode, has a value and duration less than that which will initiate adisruptive discharge of an energy level so high as to be objectionable.

l1. A method of electrostatically spray coating articles in anelectrostatic depositing field created by a unidirectional highpotential between a charging electrode and another ,electrodesubstantially at ground potential and comprising at least one of thearticles and wherein the distance between said charging electrode andsaid other electrode being subject to variations which could in aconventional electrostatic spray coating system cause an objectionablechange in potential gradient, comprising providing a distributedresistance in a rotating atomizing 25 device suiiicient to limit theeffective capacity to not in excess of that of a metal sphere of aradius of about three centimeters, flowing a stream of liquid coatingmaterial having some conductivity but relatively high resistance to theatomizing device, spreading it out in a thin annular film thereon,atomizing spray particles into said field from the edge of said filmwhereby said particles re charged to affect their deposition on thearticles while still in liquid state, providing lresistance meansincluding said distributed resistance `from the edge of the film tothehigh potential source of at least several megohms per kilovoit, andsupplying a voltage substantially in excess f the voltage which wouldotherwise be optimum in relation to the distance between electrodes atat least the lesser distances during operation.

l2. A method of electrostatically spray coating articles in anelectrostatic field created by a unidirectional high potential between acharging electrode and another electrode comprising at least one of thearticles and wherein the distance between said charging electrode andsaid other velectrode being subject to variations which could in aconventional `electrostatic spray coating system cause an objectionablechange in potential gradient, comprising iiowing la stream of liquidcoating material to an atomizing device, atomizing spray particles intosaid vield, whereby `said particles are charged to aiect theirdeposition on the articles, and automatically continuously changing thevoltage between the electrodes as a direct function of changes in thedistance therebetween to materially minimize variations in the averagefield gradient which would otherwise accompany such changes in distance,while maintaining the energy level of 4a discharge from the chargingelectrode below .that which would be provided by a metal sphere of aradius of about three centimeters in the same electrostatic system.

13. A method of electrostatically spray coating articles ina highpotential electrostatic depositing iield or" at least forty thousandvolts between a charging electrode and another electrode comprising atleast one of the articles and wherein the distance between said chargingelectrode and said other electrode being subject to variations whichcould in a conventional electrostatic spray coating system cause `anobjectionable change in potential gradient, comprising flowing a streamof liquid coating material to an atomizing device, atomizing sprayparticles into said eld therefrom, whereby said particles are charged toaffect their deposition on the articles, providing a connection to thecharging electrode having a high resistance of at least several hundredrnegohms and having an extent of at least several inches immediatelyadjacent the electrode, and applying to the kend of `said resistanceconnection farthest from the charging electrode a unidirectionalpotential higher than the potential which would cause sparking at theminimum gap distance between electrodes which would be encountered innormal use of the spray coating system, the charging electrode having aneffective capacity not greater than that of a metal sphere of a radiusof about one centimeter.

i4. ln an electrostatic coating system wherein liquid' coating materialis atomized in response to a unidirectional voltage gradient between anarticle and an atomizing electrode and the atomized coating material is,moved toward the article in response to said voltage gradient, wherein avalue of said gradient having an optimum atomizing ctiect exists foreach spacing between said article and `said electrode, and means forautomatically adjusting said gradient for optimizing atomizing eflect inaccordance with variations in spacing.

15. Apparatus for electrostatically depositing vliquid coating materialon an article, comprising, a source of unidirectional high potential; acharging electrode electrically connected to said source; means forsupporting the article to be coated; means connecting said article incircuit with said high potential source to establish an electrostaticfield between said charging electrode and said article, said circuitincluding high resistance means; a source of liquid coating material;and means for atomizing said liquid coating material into saidelectrostatic field, the distance between said charging electrode andthe article being subject to variations which could in a conventionalelectrostatic spray coating system cause an objectionable change inpotential gradient, said charging electrode being composed of suchmaterial and so dimensioned that its effective capacity is not greaterthan that of a metal sphere of a radius of about three centimeters,whereby at all distances objectionable discharge of electrical energy ofa character to initiate a re or shock to the operator is avoided.

16. Apparatus of the character claimed in claim l5, wherein saideffective capacity is not greater than that of a metal sphere of aradius of about one centimeter.

17. Apparatus for electrostatically depositing liquid coating materialon an article, comprising, a source of unidirectional high potential; acharging electrode electrically connected to said source; means forsupporting the article to be coated; means connecting said article incircuit with said high potential source to establish an electrostaticfield between said charging electrode and said article; and means foratomizing said liquid coating material into said electrostatic field,the distance between said charging :electrode and the article beingsubject to variations which could in a conventional electrostatic spraycoating system cause an objectionable change in potential gradient andsaid charging electrode vbeingcomposed ot such material and sodimensioned that its efiective capacity is not greater 'than that of ametal sphere ot a radius of about three centimeters, said effectivecapacity and the potential applied thereto being such that at alldistances between the charging electrode and article objectionabledischarge of electrical energy of a character to initiate a fire orshock to the operator is avoided.

18. Apparatus of the character claimed in claim 17, wherein saideffective capacity is not greater than that of a metal sphere of aradius of about one centimeter.

19. A method of electrostatically spray coating articles in anelectrostatic field created by a unidirectional high potential between acharging electrode and another electrode comprising at least one of thearticles and wherein the distance between said charging electrode andsaid other electrode being subject to Ivariations which could in aconventional electrostatic spray coating system cause an objectionablechange in potential gradient, comprising flowing a stream of liquidcoating material to an atomizing device and atomizing spray particlesinto said field, whereby said particles `are charged to affect theirdeposition on the articles, and automatically continuously changing thevoltage between the electrodes, by means including a high resistance, byand as a function of changes in space current across said distance tomaterially minimize variations in the average iield gradient which wouldotherwise accompany such variation in distance, while maintaining theenergy level of a discharge from the charging electrode below that whichwouldbe provided by a metal sphere of a radius of about threecentimeters in the same electrostatic system.

20. In an electrostatic system for spray coating an article `w'herein anatomizing and charging device adapted to be held in the hand has a highpotential electrostatic eld between it and the article and sprayparticles are electrostatically attracted toward the article anddeposited thereon Wh-ile still in liquid state, an atomizing andcharging device comprising: a portion on said device and adapted to facetoward the article, said portion being comprised substantially entirelyof non-conductive material and having a very low effective capacity, notgreater than that of a metal sphere of a radius of about threecentimeters, the distance between said device and the article beingsubject to variations which could in a conventional electrostatic spraycoating system cause an objectionable change in potential gradient;means adapted to be conspaanse 2? nected to a voltage supply forcreating a high unidirectional potential difference between said portionand the Vclosest article portion to create a depositing eld, thepotential creating means including high resistance means having at leasta substantial part thereof adjacent said portion and having a resistanceof at least several megohms per kilovolt of the voltage applied to thedevice; and means for supplying a controlled ilow of liquid coatingmaterial to said yatomizing device for atomization therefrom into thedepositing held, the voltage supply from the voltage source being inexcess of the voltage which could be used at at least the lesserdistances between the atomizing device and the article in the'absence ofsaid high resistance means and its effect on changes in the averagepotential gradient.

2l. Apparatus of the character claimed in claim 210, wherein the portionadapted to face toward the article comprises a rotating member of veryhigh resistivity.

22. In an electrostatic system for spray coating an article wherein anatomizing and charging device adapted to be held in the hand has a highpotential electrostatic field between it and the article `and sprayparticles are electrostatically attracted toward the article anddeposited thereon while still in liquid state, the atomizing andcharging device comprising: a portion on said device and adapted to facetoward the article, said portion being comprised substantially ofnon-conductive material and having a very low erective capacity, notgreater than that of a metal sphere of a radius of yabout onecentimeter, the distance between said device and the article beingsubject to variations which could in a conventional electrostatic spraycoating system cause an objectionable change in potential gradient;means adapted to be connected to a voltage supply for creating a highunidirectional potential difference between said portion `and theclosest article portion to create a depositing eld, the potentialcreating means including high resistance means having a megohmresistance of at least the same order as the kilovolts of the voltage`'applied to the device; and means for supplying a controlled flow ofliquid coating material to said atomizing device for atomizationtherefrom into the depositing held, the voltage supplied from thevoltage source being in excess of the voltage which could be used at atleast the lesser distances between the atomizing device and the larticlein the absence of said high resistance means and its elect on changes-in the average potential gradient.

23. Apparatus for electrostatically depositing liquid coating materialon an article, comprising, a source of unidirectional high potential; acharging electrode electrically connected to said source; grounded meansfor supporting the article to be coated; means connecting said articlein circuit with said high potential source to establish an electrostaticiield between said charging electrode and said article, .said circuithaving a high effective impedance and including high resistance means ofat least one megohm per kilovolt adjacent said charging electrode; asource of liquid coating material; and means for atomizing said liquidcoating material into said electrostatic held, the distance between saidcharging electrode and the article being subject to variations whichcould in a conventional electrostatic spray coating system cause anobjectionable change in potential gradient, said high effectiveimpedance minimizing vari-ations in average potential gradient of saidfield during relative movement between said article and electrode, saidcharging electrode being composed of such material and so dimensionedthat its eective capacity is not greater than that of a metal sphere ofa radius of about three centimeters, whereby at all distances betweenthe charging electrode and article objectionable discharge of electricalenergy of a character to initiate ya lire or shock to the operator isavoided.

24. Apparatus for electrostatieally depositing liquid coating materialon an ar-ticle, comprising, a source of unidirectional high potential; acharging electrode elec- 2d trically connected to said source; groundedmeans for supporting the article to be coated; means connecting saidarticle in circuit with said high potential source to establish anelectrostatic ield between said charging electrode and said article,said circuit having a high effective impedance and including highresistance means of at least 4() megohms adjacent said chargingelectrode; a source of liquid coating material; and means for atomizingsaid liquid coating material into said electrostatic field, the distancebetween said charging electrode and the article being subject tovariations which could in a conventional electrostatic spray coatingsystem cause an objectionable change in potential gradient, said higheffective impedance minimizing variations in average potential gradientof said ield during relative movement between said article and electrodeand being provided at least in part within the source of high potential,said charging electrode being composed of such material and sodimensioned that its effective capacity is not greater than that of ametal sphere of a radius of about one centimeter, whereby at alldistances between the charging electrode and article objectionabledischarge of electrical energy of a character to initiate a tire orshock to the operator is avoided.

25. Apparatus for electrostatically depositing liquid coating materialon an article, comprising, a source of unidirectional high potential; Iacharging electrode electnically connected to said source; grounded meansfor supporting the article to be coated; means connecting said articlein circuit with said high potential source to establish an electrostaticfield between said charging electrode and said article, said circuithaving a high effective impedance and including high resistance means ofat least 40 megohms adjacent said charging electrode; a source of liquidcoating material; and means for atomizing said liquid coating materialinto said electrostatic iield, the distance between said chargingelectrode and the article being subject to variations which could in laconventional electrostatic spray coating system cause an objectionablechange in potential gradient, said high effective impedance minimizingvariations in average potential gradient of said field `during relativemovement between said article and electrode, said high potential sourcedropping its output voltage markedly upon increases in output current,said charging electrode being composed of such material and sodimensioned that its effective capacity is not greater than that of ametal sphere of a radius of about one centimeter, whereby at alldistances between the charging electrode 'and `article objectionabledischarge of electrical energy of a character to initiate a re or shockto the operator is avoided.

26. Apparatus for electrostatically depositing liquid coating materialon an article, comprising, a source of unidirectional high potentialcomprising a ladder-type voltage-multiplying rectifier circuit suppliedwith alternating current through Ia reactive input system; a. chargingelectrode electrically connected to said source; grounded means forsupporting the article to be coated; means connecting said article incircuit with said high potential source to establish an electrostaticiield between said charging electrode and said article, said circuithaving a high elective impedance and including high resistance means ofat least 40 megohms adjacent said charging electrode; a source of liquidcoating material; yand means for atomizing said liquid coating materialinto said electrostatic field, the distance between said chargingelectrode and the article being subject to variations Vwhich could in aconventional electrostatic spray coating system cause an objectionablechange in potential gradient, said high effective impedance minimizingvariations in average potential gradient of said iield during relativemovement between said article and electrode, said charging electrodebeing composed of such material and so dirnensioned that its effectivecapacity is not greater than that of a metal sphere of a radius of aboutthree centimeters, whereby at `all distances between the chargingelectrode and article 29 objectionable discharge of electrical energy ofa character to initiate a fire or shock yto the operator is avoided.

27. Apparatus for electrostatically depositing liquid coating materialon an article, comprising, a source of unidirectional high potentialcomprising 'a ladder-type voltage-multiplying rectifier ycircuitsupplied with alternating current through an inductance capacitancebridge input network; a c'harging electrode electrically connected tosaid source; grounded means for supporting the article to be coated;means connecting said `article in circuit with said high potentialsource to establish an electrostatic field between said chargingelectrode and said article, said circuit havinga high effectiveimpedance and including high resistance means adjacent said chargingelectrode; a source of liquid coating material; and means for atomizingsaid liquid coating material into said electrostatic eld, the distancebetween said charging electrode and the article being subject tovariations which could in a conventional electrostatic spray coatingsystem cause an objectionable change in potential gradient, said highresistance means having a megohm resistance of at least the same orderas the kilovolts of -Voltage applied to the charging electrode at thelesser distances to be encountered during said relative movement, saidhigh effective impedance minimizing Variations in average potentialgradient of said eld dur-ing relative movement between said article andelectrode, said charging electrode being composed of such material andso dimensioned that its effective capacity is not greater than. that ofa metal sphere of a radius of about one centimeter, whereby at alldistances between the charging electrode and article objectionabledischarge of electrical energy of a character' to initiate a tire orshock to the operator is avoided.

References Cited in the tile of this patent UNITED STATES PATENTS1,553,364 yChubb Sept. 15, 1925 1,974,328 Bouwers Sept. 18, 19341,992,908 Cockcroft Feb. 26, 1935 2,334,648 Ransburg et al. Nov. 16,1943 2,415,116 Stiefel Feb. 4, 1947 2,509,277 Ransburg etal May 30, 19502,526,763 Miller Oct. 24, 1950 2,754,225 Gfeller July l0, 1956 2,764,125Iuvinall Sept. 25, 1956 2,764,712 Juvinall Sept.k 25, 1956 2,878,143Juvinall Mar. 17, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. 3,048,498 August 7, 1962 James W. Juvinall et al.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 2, line 53, after "that" insert if line 58, after "cusps" inserta comma; line 70, for "objectionable" read objectionably column line 38,for "which" read with lin@ 58, for "leadnig" read leading column l1,line 34, for "of energy read or energy column 16, line 65, for "used,"read used. column l', line 19, for "the "sharp configuration" read the"sharp" configuration line 30, for "cenlimeters' radius" readcentimeters radius 4column 2l, line 30, for "designated" read designedSigned and sealed this 19th day of March 1963.

(SEAL) Attest:

Es'ron G. JOHNSON DAVID L. LADD Attesting Officer Commissioner ofPatents

19. A METHOD OF ELECTROSTATICALLY SPRAY COATING ARTICLES IN ANELECTROSTATIC FIELD CREATED BY A UNIDIRECTIONAL HIGH POTENTIAL BETWEEN ACHARGING ELECTRODE AND ANOTHER ELECTRODE COMPRISING AT LEAST ONE OF ATHEARTICLES AND WHEREIN THE DISTANCE BETWEEN SAID CHARGING ELCTRODE ANDSAID OTHER ELECTRODE BEING SUBJECTED TO VARIATIONS WHICH COULD IN ACONVENTIONAL ELECTROSTATIC SPRAY COATING SYSTEM CAUSE AN OBJECTIONABLECHANGE IN POTENTIAL GRADIENT, COMPRISING FLOWING A STREAM OF LIQUIDCOATING MATERIAL TO AN ATOMIZING DEVICE AND ATOMIZING SPRAY PARTICLESINTO SAID FIELD, WHEREBY SAID PARTICLES ARE CHARGED TO AFFECT THEIRDEPOSITION ON THE ARTICLES, AND AUTOMATICALLY CONTINUOUSLY CHANGING THEVOLTAGE BETWEEN THE ELECTRODES, BY MEANS INCLUDING A HIGH RESISTANCE, BYAND AS A FUNCTION OF CHANGES IN SPACE CURRENT ACROSS SAID DISTANCE TOMATERIALLY MINIMIZE VARIATIONS IN THE AVERAGE FIELD GRADIENT WHICH WOULDOTHERWISE ACCOMPANY SUCH VARIATION IN DISTANCE, WHILE MAINTAINING THEENERGY LEVEL OF A DISCHARGE FROM THE CHARGING ELECTRODE BELOW THAT WHICHWOULD BE PROVIDED BY A METAL SPHERE OF A RADIUS OF ABOUT THREECENTIMETERS IN THE SAME ELECTROSTATIC SYSTEM.